--- a/browser/components/extensions/test/browser/browser.ini
+++ b/browser/components/extensions/test/browser/browser.ini
@@ -71,16 +71,17 @@ support-files =
 [browser_ext_tabs_query.js]
 [browser_ext_tabs_reload.js]
 [browser_ext_tabs_reload_bypass_cache.js]
 [browser_ext_tabs_sendMessage.js]
 [browser_ext_tabs_update.js]
 [browser_ext_tabs_zoom.js]
 [browser_ext_tabs_update_url.js]
 [browser_ext_topwindowid.js]
+[browser_ext_webNavigation_frameId0.js]
 [browser_ext_webNavigation_getFrames.js]
 [browser_ext_webNavigation_urlbar_transitions.js]
 [browser_ext_windows.js]
 [browser_ext_windows_create.js]
 tags = fullscreen
 [browser_ext_windows_create_tabId.js]
 [browser_ext_windows_events.js]
 [browser_ext_windows_size.js]

new file mode 100644
--- /dev/null
+++ b/browser/components/extensions/test/browser/browser_ext_webNavigation_frameId0.js
@@ -0,0 +1,47 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+"use strict";
+
+add_task(function* webNavigation_getFrameId_of_existing_main_frame() {
+  // Whether the frame ID in the extension API is 0 is determined by a map that
+  // is maintained by |Frames| in ExtensionManagement.jsm. This map is filled
+  // using data from content processes. But if ExtensionManagement.jsm is not
+  // imported, then the "Extension:TopWindowID" message gets lost.
+  // As a result, if the state is not synchronized again, the webNavigation API
+  // will mistakenly report a non-zero frame ID for top-level frames.
+  //
+  // If you want to be absolutely sure that the frame ID is correct, don't open
+  // tabs before starting an extension, or explicitly load the module in the
+  // main process:
+  // Cu.import("resource://gre/modules/ExtensionManagement.jsm", {});
+  //
+  // Or simply run the test again.
+  const BASE = "http://mochi.test:8888/browser/browser/components/extensions/test/browser/";
+  const DUMMY_URL = BASE + "file_dummy.html";
+  let tab = yield BrowserTestUtils.openNewForegroundTab(gBrowser, DUMMY_URL, true);
+
+  function background(DUMMY_URL) {
+    browser.tabs.query({active: true, currentWindow: true}).then(tabs => {
+      return browser.webNavigation.getAllFrames({tabId: tabs[0].id});
+    }).then(frames => {
+      browser.test.assertEq(1, frames.length, "The dummy page has one frame");
+      browser.test.assertEq(0, frames[0].frameId, "Main frame's ID must be 0");
+      browser.test.assertEq(DUMMY_URL, frames[0].url, "Main frame URL must match");
+      browser.test.notifyPass("frameId checked");
+    });
+  }
+
+  let extension = ExtensionTestUtils.loadExtension({
+    manifest: {
+      "permissions": ["webNavigation"],
+    },
+
+    background: `(${background})(${JSON.stringify(DUMMY_URL)});`,
+  });
+
+  yield extension.startup();
+  yield extension.awaitFinish("frameId checked");
+  yield extension.unload();
+
+  yield BrowserTestUtils.removeTab(tab);
+});